The short story is that U.S. companies can, with a certain amount of effort, move their legal address to Ireland or wherever and more or less avoid paying any taxes. This is called an "inversion," and the math is pretty straightforward.

Cheap is converging with expensive in the American equity market, narrowing options for investors looking for bargains after the broadest rally on record lifted almost 90% of the Standard & Poor’s 500 Index this year.